1. Field
This invention relates to devices for the high quality reproduction of sound and particularly to those that use a resonant enclosure to allow the rear radiation of the driver to reinforce the direct radiation at low frequencies.
2. Description of Prior Art
Two considerations dominate the design of a loudspeaker required to accurately reproduce low frequencies. A large amount of air must be displaced by the motion of the speaker cone, and the pressure field produced by the displacement of the rear of loudspeaker must not be allowed to cancel that produced by the front. In addition the loudspeaker cone exhibits a mass and its suspension exhibits a compliance. These together determine a resonant frequency below which the sound output drops rapidly.
A common solution is to use a loudspeaker sufficient in size to displace enough air and to mount it in a closed box so that front to rear cancellation cannot occur. The compliance of the air in the box raises the resonant frequency of the speaker so that if a desirably low response is to be maintained, either a large box must be used, or the mass of the speaker cone must be increased. If the mass of the speaker is increased, the efficiency of conversion of electrical to acoustic energy will be lowered.
Many designs use the pressure field produced by the rear of the loudspeaker to enhance the low frequency response through various arrangements of resonators, ducts or horns. Those using ducts or horns are not directly related to this invention, although the rear horn loaded type exhibits many of the desirable properties of this invention, but at some cost in size and complexity (Hancock, 1975, U.S. Pat. No. 3,923,124).
Most directly related to this invention are those speaker systems which turn the closed box into a resonator by providing a mass loaded opening in the enclosure. The mass associated with the opening resonates with the compliance of the air in the box in such a way that when the resonator is excited by the motion of the rear of the loudspeaker cone the pressure field emitted through the opening is in phase with that produced by the front of the loudspeaker and cancellation does not occur. Thus both sides of the speaker do useful work and the efficiency of the loudspeaker is increased. For a general discussion of resonator or bass reflex enclosures see L. L. Beranek, Acoustics, McGraw-Hill, 1954.
The mass loading of the opening can be produced by the acoustic mass associated with the opening. This acoustic mass may also be augmented by the use of a duct. Alternately, the mass can be produced by a tangible diaphram flexibly suspended in the opening. In this case there is an additional compliance associated with the suspension of this passive radiator, but operation is substanially the same as with acoustic loading (H. F. Olson, 1935, U.S. Pat. No. 1,988,250).
A resonant enclosure loudspeaker can produce useful sound output down to the free-air resonant frequency of the speaker. This requires, however, an enclosure with compliance at least as large as that of the speaker alone. For a practical speaker large enough to reproduce low frequencies the enclosure required is quite sizable. If the enclosure size is reduced either the low frequency response or the efficiency must suffer. Thus good bass response with a speaker large enough to insure low distortion and good efficiency of conversion of electrical to acoustic energy has heretofore required a relatively large enclosre.
The invention disclosed herein is an improved form of resonant enclosure which is of smaller size than previous designs for a given combination of distortion, frequency response and efficiency.